The inner ear is the innermost part of the ear. As shown in FIGS. 1A and 1B, sound is directed by the pinna 102 through the ear canal 104 to the eardrum 106. The eardrum 106 moves the bones of the middle ear 108 to vibrate the cochlea. The cochlea generates electric pulses that are correlated with the sound, and these electric pulses are sent to the brain. The inner ear further includes a balance sensing system 110, referred to as the vestibular system. The vestibular system 110 generally includes three semicircular canals 112 and two pairs of otolithic organs (each located on a different side of the head). Each pair of otolithic organs includes a utricle 116 and a saccule 118. Internal to the semicircular canals 112 and surrounding the otolithic organs are the endolymphatic ducts containing endolymph. Multiple ampullae 120 may also be disposed in the inner ear. The semicircular canals 112 may be characterized as providing three rotational receptors (the ampullae 120) and two gravitational receptors (the otolithic organs 116, 118). The semicircular canals 112 and the otolithic organs 116, 118 in the inner ear contain hair-cell transduction mechanisms that, for example, help (i) provide the brain with spatial orientation cues, (ii) keep the eyes focused on a target when the head is in motion, and (iii) maintain balance. Specifically, the ampullae 120 of the semicircular canal 112 respond to rotations, while the otolithic organs 116, 118 sense linear accelerations, decelerations, and tilting. As a result, stimulations of normal otolithic organs, specifically, the utricle 116 and saccule 118, will produce a response in (i) the eye muscles to allow the eyes to maintain gaze and (ii) the muscles that contribute to movement of the head.
Gravitational receptor asymmetry produces dizziness, a sense of motion, tilting, being pushed, pulled or falling; while rotational receptor asymmetry produces true rotational vertigo. Ninety million Americans go to health care providers because of vertigo, dizziness, or balance problems. It is the second most common complaint heard in doctor's offices, and will occur in 70% of the nation's population at some time in their lives. Falls account for 50% of accidental deaths in the elderly, and 10% of falls result in hospitalization. Every 15 seconds, an older adult is treated in the emergency room for a fall; every 29 minutes, an adult dies following a fall. Research has indicated that the annual direct and indirect costs of fall-related injuries are estimated to reach $54.9 billion by the year 2020, and that participants with vestibular dysfunction who were symptomatic, i.e., reported dizziness, independently increased the odds of falling more than 12-fold. Research has also indicated that increasing age is associated with an increased prevalence of vestibular dysfunction. There are also military considerations with post combat-induced injuries and loss of military aircraft and other assets that contribute to the scope of vestibular related problems.
It is known that bone-conducted stimulation to the head, as well as auditory stimulation, excites the otolithic organs 116, 118. As a result of the stimulation, a response (e.g., action potential) is produced at the sternocleidomastoid muscle (a neck muscle that contributes to the movement of the head) and the extraocular muscles (eye muscles that allow the eyes to move and focus). In addition, inhibitory or excitatory action potentials are produced at other muscles (e.g., triceps or splenius capitus muscles) in response to activation of the two otolithic organs. A muscle response may be characterized or measured as an electrical impulse from the brain to the muscles. Specifically, a cervical vestibular evoked myogenic potential (cVEMP) response has been observed to be an inhibitory response, measured at the sternocleidomastoid muscle, corresponding to an activation of the saccule 118. Also, an ocular vestibular evoked myogenic potential (oVEMP) response has been observed to be an excitatory response, measured at the inferior oblique muscle (an extraocular muscle that controls a specific movement of the eyes), corresponding to an activation of the utricle 116. The extraocular muscles include six muscles, including the inferior oblique, superior oblique, medial rectus, superior rectus, inferior rectus, and the lateral rectus muscles. By observing the cVEMP and oVEMP response, it is generally known that diseases, disorders, and conditions affecting the vestibular system and the balance sensing system of a person may be observed.
Besides producing auditory stimuli, some in the art have developed various types of apparatus to deliver bone conduction stimuli to certain parts of the skull bone (e.g., the frontal bone, the parietal bone, the occipital bone) to test the gravitational receptor functions of the inner ear. For example, some in the art have employed electromechanical devices and mini-shaker apparatus. Such apparatus have been observed to produce stimuli of insufficient magnitude to elicit a robust response. Some in the art have also employed solenoid actuators. Existing arrangements including solenoid actuators may produce stimuli of sufficient magnitude, but may also produce other stimulations of the otolithic organs for certain patients. Some have used reflex hammers. Of these conventional approaches, the reflex hammer tapping of the forehead may produce the most robust cVEMP and oVEMP responses, but there is no mechanism to standardize and calibrate the stimulus.
Bone conduction stimulus may be applied at various locations on the head, for example, at the front of the forehead along the mid line. FIGS. 1C and 1D illustrate example electrode placement diagrams for delivery of bone conduction stimuli. The bone conduction stimuli may be applied at the same location among different patients and among different tests in order to improve the repeatability of the testing, for example, at the “Fz” location 120.